As a known measure for protecting so-called thin film modules against so-called TCO corrosion it is described in the document DE 20 2006 008 936 U1 to ground the negative connection terminal of a photovoltaic generator, or to connect it to the neutral conductor of a connected AC power grid, such that the solar modules may not have a negative electric potential with respect to ground and thus with respect to their environment. The disclosure in the document DE 20 2006 008 936 U1 is described as to even raise the negative connection terminal of the photovoltaic generator to a positive potential with respect to ground. For this purpose, a bias voltage source shall be arranged in a bypass line between the neutral or protective conductor of the AC power grid and the negative connection terminal of the photovoltaic generator. Both the direct connection of the negative connection terminal of a photovoltaic generator to the neutral conductor of an AC power grid and the in-between-arrangement of a bias voltage source that raises the negative connection terminal to a positive potential with respect to the neutral conductor requires an inverter with galvanic separation between the photovoltaic generator and the AC power grid.
It is known that inverters may comprise a transformer for galvanic separation. If this transformer is provided for AC current that is output at grid frequency and for transferring a higher electric power, it is a large and particularly heavy, as well as expensive component. Thus, often HF transformers as parts of DC/DC converters are used for galvanic separation. In this case the transformers themselves may be smaller. However, additional switching effort has to be spent.
A transformerless inverter comprising a DC/DC converter that is connected to the input side of an inverter bridge is known from the documents WO 2010/069620 A1 and US 2010/0008105 A1, which belong to the same patent family. The DC/DC converter comprises series resonant circuits with resonance capacitances that provide for a galvanic separation between the input lines of the inverter and the input lines of the inverter bridge. In this way, it is possible for a photovoltaic generator connected to the input lines of the inverter to select the potential with respect to ground freely by connecting any point of the photovoltaic generator in an electrically conductive way to ground or to a reference potential defined with respect to ground. Although no transformer is required for this purpose, nevertheless the additional DC/DC converter at the input side of the inverter bridge is needed. Due to selecting the potential of the photovoltaic generator, the resonant capacitances will be charged to a DC voltage, which is, however, not mentioned in the documents.
An inverter that has input lines provided for connection of a photovoltaic generator and that lead directly to an inverter bridge is known from Klumpner, C., et al. “A More Efficient Current Source Inverter with Series Connected AC Capacitors for Photovoltaik and Fuel Cell Applications” Proceedings PCIM Europe 2009 Conference, pages 153 to 158, ISBN: 978-3-8007-3158-9. Capacitors are arranged in the output lines of the inverter bridge. However, it is not intended to charge these capacitors to a DC voltage. The document does not at all concern the definition of an electric potential of the input lines of the inverter bridge and thus of the connected photovoltaic generator with respect to ground.